The international publication WO99/39368 discloses a mass spectrometer including the elements of: an ion trap device as an ion source; a flight space for letting ions from the ion source fly straight; a reflector for reflecting back the ions flying the flight space using an electric field; and a detector for detecting ions that have flown the flight space. The ion trap device is composed of a ring electrode and a pair of end cap electrodes placed opposite to each other with the ring electrode therebetween. Applying appropriate voltages to the electrodes, a three-dimensional quadrupole electric field is generated in the space (ion trap space) surrounded by the electrodes, where ions are stored, or ions of a specific mass to charge ratio or ratios are selected. Further, by introducing an appropriate collision gas in the ion trap space, ions can be dissociated (Collision Induced Dissociation, CID). Besides such a three-dimensional quadrupole ion trap device, a quadrupole mass filter, such as a four-rod quadrupole filter, is known to be able to store ions.
The ion trap described above stores ions in such a manner that the trajectories of vibrating ions converge in the ion trap space due to the quadrupole electric field generated within the ion trap space. When ions of a specific mass to charge ratio are to be ejected from the ion trap space, an RF electric field whose frequency corresponds to the mass to charge ratio is generated in the ion trap space. However, it is difficult to generate a wide range RF voltage devoid of a specific frequency or a specific frequency band. It is also difficult to generate an RF voltage having a specific frequency or a specific frequency band. Therefore, the mass selectivity (or mass resolution) of ions of the above ion trap is not satisfactory, and it is difficult to adequately eliminate ions having very close (0.01 amu, for example) mass to charge ratio to object ions.